A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus, a stage apparatus is used to position an object table (e.g. holding a substrate or patterning device) relative to e.g. a metrology frame to which a projection system is mounted. Such a stage apparatus in general comprises a positioning system (e.g. comprising a long stroke coarse positioner and a short stroke fine positioner) for positioning the object table relative to the frame and a measurement system for determining the position of the object table relative to the frame with high accuracy (e.g. nanometer accuracy). Due to a continuing demand for higher throughput and increased accuracy, there is a need to improve the accuracy of measurement systems used in the lithographic apparatus, in particular for the measurement systems with which the position of the substrate stage and reticle stage are measured, and typically in six degrees of freedom.
In a known embodiment of the measurement system an encoder type measurement system is used. Such encoder-type measurement system may comprise one or more sensors mounted on the movable object and at least one sensor target object, for instance a sensor target plate comprising a grating or grid, the sensor target object mounted on a substantially stationary frame, in particular a so-called metrology frame (metro-frame). The sensor target object may comprise a one-dimensional or multi dimensional grating. The sensor target object will be typically in the form of a plate on which a two dimensional orthogonal grid is arranged. Such sensor target object is often referred to as grid plate.
In alternative embodiments, the one or more sensors may be mounted on the substantially stationary frame and the grid plate or the grid plates may be mounted on the movable object. The grid plate comprises a number of grid lines or other grid markings which are used to determine a change in position of the grid plate with respect to the one or more sensors.
A conventional measurement system comprises a mounting device for mounting the grid plate at a number of mounting points on the substantially stationary frame. Temperature changes and/or temperature differences in the metro frame may cause a change of shape of the metro-frame. Also other influences may cause shape changes of the metro-frame. As a result, the distance between the mounting points of the mounting device of the grid plate may change and, as a consequence, the change in shape of the metro-frame may also result in a change of shape of the grid plate. Such deformations may have a negative influence on the measurement accuracy of the measurement system.
In order to compensate for such shape changes of the metro-frame, the mounting device comprises a number of flexure elements which connect the grid plate to the metro-frame. These flexure elements are flexible in at least one degree of freedom in order to compensate the possible change in relative positions of the mounting points.
In a typical encoder-type measurement system for a substrate stage, a grid plate is mounted with three flexure elements which are arranged on the circumference of an imaginary circle drawn about the central axis of the lens column of the lithographic apparatus. The flexure elements allow movements of the metro-frame in radial direction with respect to the central axis of the lens column without transferring these movements to the grid plate. As the metro-frame is typically designed to deform circle-symmetrically about the central axis of the lens column, the grid plate will substantially remain in its position due to the flexure elements.
A drawback of the known measurement system is that since the mounting device of the grid plate, in particular the flexure elements, introduce some flexibility in the mounting of the grid plate, the grid plate may be moved or deformed by external influences. For instance, movements of the substrate stage may cause pressure waves which may cause movements or deformations in the grid plate. Such movements and/or deformations have a negative influence on the performance of the measurement system. In other measurement systems similar effects may occur and may have a negative influence on the accuracy of the measurement system.